Aircraft have numerous systems for conveying fluids, such as air, water, fuel, coolants, and hydraulic fluid as examples. Couplings or retention devices within these systems may include rigid piping, or flexible piping such as tubes, hoses, couplings, and the like. The fluids and systems in which these couplings are used may include a wide range of temperatures and pressures. Exemplary temperatures may range from −70° F. (−56.6° C.) to 275° F. (135° C.), and exemplary pressures may range from −14.5 psig (−0.7 kPa) to +300 psig (14.4 kPa).
To prevent electrical static buildup and also to convey high levels of electrical current during a lightning strike, electrical current flow within fluid conveyance systems is encouraged by designing the components having electrically conductive materials. The fluid conveyance components and systems are electrically bonded to the aircraft's electrical grounding plane. The electrical bonding is typically achieved through the connecting of these fluid conveying components using highly conductive metallic bonding devices including wires, strips, and straps.
The evolution from metallic skinned aircraft to composite skinned aircraft has influenced aircraft manufacturers to replace the highly conductive metallic fluid conveyance systems with composite based systems. These composite systems can include fully composite components or a hybrid of composite and metallic components. The composite components in either system meet electrical conductivity specifications driven by the aircraft's geometry and materials of construction.
The electrical conductivity specification of the fluid conveyance systems for composite aircraft permits replacing the metallic bonding devices with conductive seals for electrical bonding between components. Seals within these systems therefore perform over a wide variety of temperatures and pressures, and are typically used to seal against a wide variety of fluids, while meeting demanding electrical conductivity requirements.
The current conductive seal technology as used as a bonding device meets the performance specifications for specific fluid conveyance systems. However, this current conductive seal technology does not meet the higher voltage lightning specifications for all fluid conveyance systems of the aircraft. It is desirable that these fluid conveyance applications and the conductive seals safely conduct higher potential lightning currents without arcing between components and or between components and aircraft structure.
Arcing has been observed to occur on an outside surface of the conductive seal between highly conductive sealing surfaces at the higher voltage lightning pulses. Reasons for arcing on the outside surface may include, for instance, low surface electrical resistance, ionization of the seal surface due to high current potential coupled with the lightning wave form, high volumetric resistance, very high current density, as examples. Ionization of the seal surface is typically where the initiation or creation of the electrical current path begins.
Some of the seals within aircraft are non-electrically conductive. Thus, at times it is possible for aircraft operator service personnel to place a non-electrically conductive seal within a system, where a conductive seal is required. Such systems would lose their electrical bonding which protects against lightning strikes, as well as static electrical buildup and the dangers inherent therewith.
As such, it would be desirable to have a seal for a fluid conveying system in an aircraft that is distinguishable from non-conductive seals, and resists seal surface ionization when passing a very high current density from a lightning strike.